1. Field of the Invention
This invention relates to an analog-to-digital converter of the pulse-width modulation type, and particularly to an incremental pulse width modulation type for generating a digital representation of the amplitude of an input analog current which minimizes offset and time lag errors.
2. Description of the Prior Art
This invention is an improvement over U.S. Pat. No. 3,918,050, entitled "ANALOG-TO-DIGITAL CONVERSION APPARATUS" and U.S. Pat. No. 4,112,428, entitled "CLOCKED PRECISION INTEGRATING ANALOG-TO-DIGITAL CONVERTER SYSTEM", both by A. K. Dorsman, and assigned to the same common assignee. In earlier systems employing integrating analog-to-digital converter systems of the type disclosed in the two patents above-mentioned, such systems were used in strap down accelerometer applications using digitizer (analog-to-digital conversion) circuits. Both velocity accuracy and response time characteristics were adequate to meet the requirements of these less severe acceleration environments. Typically, though sizeable velocity errors did exist, these systems performed adequately. However, in higher accuracy applications the velocity error became significant and to first order was proportional to the integrator voltage at the time of sampling, i.e., at the measurement time. In particular, one of the voltages contributing to the integrator output voltage was either increasing or decreasing in proportion to the input acceleration. A significant velocity error with a time lag was the result.